In 1968 he began working as a lawyer for a firm on Wall Street, moving to the World Bank the following year, where he worked on investments in Africa and the Middle East.[1] In 1975 he returned to Greece to take up the post of deputy governor of the Hellenic Industrial Development Bank (ETVA), while also being a member of the committee preparing Greek accession to the EEC.[1]

From 1977 he was active in Greek politics as a member of the conservative New Democracy party, being elected as an MP on the nation-wide list.[1] He was elected to the Greek parliament ten consecutive times from that year.

He went on to hold a variety of ministerial and other political positions:[1]

Dimas served briefly in the Prodi Commission. He was appointed European Commissioner for Employment and Social Affairs in March 2004, taking over the role from the previous Greek Commissioner Anna Diamantopoulou. A major focus of his work during this time involved making it more difficult for states to opt out of the Working Time Directive. The Greek government nominated Dimas for the incoming Barroso Commission which took office on 22 November 2004.

Concerns were raised in advance by MEPs and many NGOs that by appointing a former Wall Street lawyer and industry minister to the environment post the EU had signalled a decreased commitment to environmental issues in favour of promoting economic competitiveness. However, after some months time, the environment unions quickly endorsed his programme and publicly congratulated him for being able to support the positions of the environmental organizations with great success within the Commission. Dimas' role was vital especially since the President, Mr Barroso, was ready to disregard the effects to the environment due to the need to improve economic development.

Questioned by the European Parliament, Dimas took the view that preserving the environment and promoting competitiveness went hand in hand, and that environmental policy could provide a stimulus for technological innovation. Among other statements, he said that GM residues in seeds should be the lowest technically feasible.

Following his hearing Dimas received a sceptical response from most MEPs. Many believed him to have insufficient experience to fulfill the role. Karl-Heinz Florenz, chair of the Environment Committee and a member of the conservative EPP-ED, said that European environment policy needed, "more than just declarations of intention.... If Commission President-designate Barroso puts the Lisbon Strategy on top of the agenda, this must not mean that the primacy of economic over environment policy gets cemented." However, he stressed the need to give Dimas time to 'grow into' his role. The Socialists gave him a qualified approval but criticised his plans as too vague. The Greens and the left-wing GUE/NGL opposed his appointment outright, with the Greens naming him as one of the three "incompetent" Commissioners-designate who prompted them to vote against the Commission as a whole. Representatives of industry and the business community welcomed his appointment to the environment post.

In a dramatic change of stance, the Greens endorsed the programme plan by Commissioner Dimas and have argued that their cooperation in environmental issues is harmonic.

Dimas led the EU in its tough line on climate change during the first weeks of the new Commission. At UN talks on climate change in Buenos Aires in December 2004 he attempted to negotiate a new system of mandatory emissions reductions to follow the expiration of the initial Kyoto targets in 2012. This approach met with fierce opposition from the U.S., representatives of which refused to even discuss the matter. The Italian environment minister Altero Matteoli broke EU ranks on the issue, proposing voluntary targets after 2012, and saying that it was, "unthinkable to go ahead without the US, China and India."

In response to the refusal of countries to sign Kyoto, such as the United States and Australia (the latter of which exchanged viewpoints with the EU on the matter),[2] the EU has been looking to tax products imported from those countries not taking low-carbon policies on board (Border Tax Adjustments).[3]

When defending the Commission's new proposals to climate change in 2007, he claimed "It is clear that the fight against climate change is much more than a battle. It is a world war that will last for many years."[4]

Dimas oversaw the introduction of the EU's emissions trading scheme, that took effect on 1 January 2005, despite emissions reduction plans from Poland, Italy, the Czech Republic and Greece not having been approved on time. He also sought to include companies operating aircraft under the emissions trading regime.

In February 2007 the Commissioner put forward his plans to increase fuel efficiency standards of cars so that emissions are no more than 130g of CO2 per km, down from 162g/km in 2005. This caused anger from the European car industry[5] which was stoked by the Commissioner requesting a Japanese car, a Toyota Prius, instead of a European make due to the Toyota's better environmental standards.[6]